The present application relates to batteries and more particularly relates to molten salt electrolyte systems that have melting temperatures from approximately room temperature to 250xc2x0 C. and higher.
There are generally two main types of high temperature electrical batteries that use molten salt electrolytes: thermal batteries and high temperature rechargeable batteries. The thermal batteries are primary (non-rechargeable) batteries and normally generate a single continuous energy output on activation. The duration of the output may vary from a few seconds to over two hours dependent on battery design and construction. Such batteries are particularly suited to short burst high power applications. The high temperature rechargeable batteries are of interest as power supplies for submarines and electric vehicles. Both primary reserve thermal batteries and high temperature rechargeable batteries use an electrolyte that is an ionically-conducting liquid in the high temperature operational state but a non-conducting solid in the storage state.
Thermal batteries are formed from a series construction of cells having an inert state and a thermally active state. The normal storage state is the inert state when the cell electrolyte between the anode and cathode is in a solid form, generally at ambient temperature. The thermally active state exists when the electrolyte is molten and this state may be obtained rapidly when required by igniting a charge of a pyrotechnic material in close proximity to the electrolyte. The cell temperature in the thermally active state is typically 350-600xc2x0 C.
In known modern thermal batteries, the anode is usually based on lithium. This may be in the form of a solid electrode of a lithium alloy (with boron, silicon or aluminum) held in a support or as liquid lithium or lithium based mixtures retained in a foraminous metal substrate by capillary action. Almost all modern thermal batteries use lithium or its alloys in the anode because of its high electrode potential, its high coulombic capacity due to its low atomic weight and its relatively high chemical stability which facilitates handling. The cathode is a disc of iron sulfide or disulfide containing electrolyte or separator materials (electrolyte-binder), and lithium oxide to suppress voltage transients on activation.
The separator which provides ionic conductivity during discharge is generally in the form of a pressed powder pellet commonly comprising a binder and a eutectic mixture of lithium chloride and potassium chloride or of lithium fluoride, lithium chloride and lithium bromide, although other mixtures are known. The electrolyte is generally incorporated into an inert binder such as magnesium oxide, zirconia, or aluminum nitride, which immobilize it when molten by capillary action. The advantages of molten salts as battery electrolytes are that they have high conductivity for high currents and power densities, thermal stability, chemical stability towards anodes and cathodes, cheapness and ready availability. However, they also have the disadvantage of having high melting points which necessitate high operating temperatures, high heat input to activate the batteries and may cause thermal management problems.
The thermal input required can be obtained rapidly by ignition of pyrotechnic material, which can be a mixture of iron and potassium perchlorate in the form of a pellet contained in each cell. The cells can be connected in a stack, each cell being separated by a pellet of pyrotechnic material. The stack is typically hermetically sealed within a stainless steel case. The high power density and long maintenance free storage life ( greater than 50 years) make thermal batteries well suited to certain military applications.
The term molten salt electrolyte as used in the context of such batteries generally refers to a lithium halide containing salt that is maintained at a temperature above its melting point. The molten salt is commonly either a mixture of lithium halides, or a mixture of one or more lithium halides in combination with other alkali metal or alkaline earth halides. In the latter case, binary eutectic mixtures of lithium halide salts with halide salts of potassium, or occasionally barium and strontium, are preferred. Two principal molten electrolytes that have become established for high temperature batteries are binary lithium chloride-potassium chloride eutectic mixtures of melting point around 352xc2x0 C. and ternary lithium fluoride-lithium chloride-lithium bromide eutectic mixtures of melting point around 445xc2x0 C.
When power sources are required for lower-temperature applications, such as in geothermal technology applications, lithium/thionyl chloride cells can be used but they are limited to operating temperatures of 180xc2x0 C. to 200xc2x0 C. Operation at higher temperatures can caused explosive or venting events, presenting hazards to personnel and adjacent electronics.
Mixtures of salts of lithium and other metals have the advantage of lower melting points than mixtures of lithium halides only, but high currents cannot be passed at temperatures only slightly above the melting point due to resultant lithium concentration changes raising the melting point. The above is particularly marked in the case of the preferred binary eutectics. The effective minimum operating temperature is therefore appreciably above the melting point of the eutectic. This problem does not arise with electrolytes comprising mixtures of lithium halides only, which offer higher conductivities at or near the melting point, but these already exhibit higher melting points and thus also require relatively high heat input to operate the batteries. The so-called all-lithium mixture also exhibits a high rate of self discharge when batteries using them are placed on open circuit.